(1) Field of the Invention
The present invention relates to a redundant structure control device for an exchange, particularly to a redundant structure control device for an exchange, capable of separately dealing with trouble that has happened at any of lines and trouble that has happened at any of line interface devices which terminates the lines, in an ATM (Asynchoronous Transmission Mode) exchange having a redundant structure of lines and line interface devices.
(2) Description of the Related Art
Conventionally, an ATM exchange has a redundant structure of lines and line interface devices wherein the ratio of lines and line interface devices for current use to spare lines and line interface devices is 1:1. When any of lines and line interface devices for current use becomes unavailable due to trouble or something, switching to a spare line or spare line interface device is performed so that the service can be continued without interruption.
Recently, in view of cost, instead of the redundant structure wherein lines and line interface devices are provided completely doubly, that is, as many spare systems as systems for current use are provided, a redundant structure has been proposed wherein a single spare system is provided to N systems for current use so that when trouble happens at any of the lines and line interface devices of the systems for current use, switching from the system at which trouble has happened to the single spare system may be performed.
Japanese Patent Preliminary Publication Hei 9-238118 discloses switching control performed when trouble has happened. According to the disclosure, a line switching device for switching from the line and line interface device of one of N systems for current use to the line and line interface device of a single spare system is provided not as a spatial switch but as a time switch so that the line switching device may not be complicated even with an increased number of channels.
Japanese Patent Preliminary Publication Hei 11-27286 discloses another way of dealing with trouble. According to the disclosure, at least one spare device for a spare line is provided in advance, and a by-pass route is set up between the spare device and each device for current use. (When a plurality of spare lines are provided, a plurality of spare systems are connected in a row with a by-pass route.) When trouble happens at a line for current use, switching to the by-pass route set up in advance is performed so that cells may be sent out through the spare line.
In any of the conventional ATM exchanges, a line and a line interface device that holds the line are integrated. Therefore, when trouble happens at either a line or a line interface device of a system for current use, switching to the line and line interface device of the spare system is performed.
In the (N+1) redundant structure, when trouble happens at either a line or a line interface device, both the line and the line interface device of the spare system are used. Therefore, for example, when trouble happens at a line, therefore, switching to the line and line interface device of the spare system is performed, and then trouble happens at another line interface device, the latter trouble cannot be dealt with. This is because there remains no available spare system, though the line interface device connected with the line at which trouble has happened first is in order.
Further, in the case where a plurality of lines are held in the same line interface device, if trouble happens at the line interface device, the service stops simultaneously at those plurality of lines. In the ATM, switching is performed on a VP (virtual path) or VC (virtual channel) connection basis. Measures to perform switching on a line basis are not provided. Therefore, if trouble happens at a line interface device, the effects thereof is serious.
Further, in the ATM exchange, an output route for each cell is usually determined by a line interface device located on the input side of an ATM switch. In that case, if trouble happens at a line interface device, all the paths that include that line interface device as an output route need to be changed to include a spare system instead. However, path information is usually stored in a plurality of line interface devices located on the opposite side of the ATM switch to the side on which the line interface device at which trouble has happened is located. Therefore, usually, for all the paths that include the line interface device at which trouble has happened as an output route, path information stored in the line interface devices on the input side needs to be changed. Thus, it takes long time to change all the paths that have connections at the line interface device at which trouble has happened. For example, when 8000 connections are held in the line interface device at which trouble has happened and it takes 10 milliseconds to rewrite path information for one path, it takes 80 seconds to change all the paths. This causes a large decrease in performance of line service.